Non-reusable hypodermic syringe

ABSTRACT

A single use disposable hypodermic syringe having a hollow elongated barrel portion, a piston and sealing engagement with an axially translatable along the barrel portion, a piston rod secured to the piston and projecting from the rearward end of the barrel portion for use in reciprocating the piston, a walled chamber disposed proximate the forward end of the barrel portion and plug means disposed in the chamber and moveable therein from a rearward position to a forward position in response to liquid being forced against the plug member by the piston. The plug member defines fluid passages therein to allow fluid flow about the plug member and an annular flange which cooperates with the interior surface of the chamber for maintaining the plug member in the rearward position upon air being passed through the fluid passages in response to translation of the piston within the barrel portion. The annular flange also cooperates with a second portion of the chamber wall for permanently maintaining the plug member in the forward position upon the plug member being moved to the forward position by the liquid during ejection thereon from the syringe. With the plug member in the forward position, the annular flange thereon seals the fluid passages upon the piston being retracted within the barrel portion toward the rearward end thereof to prevent any additional liquid from being aspirated into the barrel portion of the syringe about the plug member thereby rendering the syringe non-reusable.

This is a continuation of application(s) Ser. No. 07/478,667 filed onFeb. 12, 1990, now U.S. Pat. No. 5,254,093.

BACKGROUND OF THE INVENTION

The present invention relates to a non-reusable hypodermic syringe. Ashypodermic syringes are used for inoculating against infectious diseasesand administering prescribed medicines, the advantages of a non-reusablesyringe are obvious. With the widespread use of illicit drugs and thespread of Acquired Immune Deficiency Syndrome, commonly know as AIDS,the present need for such a device is acute. While efforts haveheretofore been made to provide such a device, as seen in U.S. Pat. Nos.4,781,684, 4,832,693 and 4,850,968, such efforts have not met withwidespread success as a result of deficiencies in the designs of themechanisms developed to render syringes non-reusable.

A non-reusable syringe should not only be tamper-proof such that themeans by which the device is rendered non-reusable can not be removed,but should also self-destruct without any cooperation of the user. Forexample, the syringe should not require a completion of the full strokeof the plunger within the barrel of the syringe to render the deviceinoperative as in the devices disclosed in the first two above-citedpatents, or one could then leave the barrel of the syringe half full andcontinue to re-fill the upper part of the barrel. Second, it isimportant that syringe be capable of acting as an air pump withoutrendering the syringe inoperative in order to maintain the interior ofthe vials of medicine at atmospheric pressure. When medicine iswithdrawn from a standard vial of medicine by a hypodermic syringe, anegative pressure is created within the vial due to the air-tight sealsused on such vials. Repeated withdrawal of medicine from the vial willbuild up a significant negative pressure within the vial. As thehypodermic needle on a syringe is withdrawn from a vial in which such anegative pressure has been created, air is aspirated into the syringethrough the needle, creating small bubbles of air in the liquid medicinewith the syringe which is extremely hazardous with intravenousinjections. In addition, many injections require a combination of twomedicines drawn from separate vials. If a negative pressure is allowedto build up within the vials, contamination and mixing problems willresult as well as air bubbles. After drawing into the syringe thedesired volume of medicine from the first vial and then inserting thesyringe needle into a second vial which has a negative pressure therein,medicine will be drawn from the syringe back through the needle into thesecond vial, contaminating the medicine in the second vial and making anaccurate measuring of the proportions of the two medicines in thesyringe extremely difficult. To avoid these problems, it is commonpractice with conventional hypodermic syringes to first draw into thesyringe a volume of air equal to the volume of medicine to be extractedfrom a particular vial and then inject that measured volume of air intothe vial. Upon subsequently withdrawing an equal volume of medicine fromthat vial, the pressure within the vial will be left at atmospheric. Inthe single use hypodermic syringes heretofore developed, the means whichrender such devices non-reusable will act to render the syringeinoperative if one were to attempt to use it as an air pump in an effortto maintain the interior of the vial at atmospheric pressure. As aresult, such devices are not only highly susceptible to the ingestion ofair, but are also illsuited for injections requiring a mixture ofmedicines.

The non-reusable hypodermic syringe disclosed herein provides a singlesolution to all of the above problems by differentiating between thedensity of fluids therein and destructing solely in response to theejection of liquid therefrom. The fluid density responsive destructmechanism of the present invention will not activate upon the passage ofair into and out of the syringe so that the syringe freely used as anair pump prior to any liquid being drawn therein to prevent negativepressure from building up in the vials of medicine. However, once aliquid has been drawn into the syringe and even a portion thereofejected, the destruct mechanism is activated and no additional fluid,air or liquid, can be drawn into the syringe, rendering the deviceinoperable other than to eject the remaining liquid within the syringe.

SUMMARY OF THE INVENTION

The present invention provides a disposable and non-reusable hypodermicsyringe. The syringe is rendered non-reusable by means of a fluiddensity responsive plug means disposed in a lower chamber of the syringewhich, prior to aspirating liquid into the syringe, allows air to freelypass thereby so that the syringe can be used as an air pump to maintainatmospheric pressure within vials of medicine. Upon liquid being drawninto the syringe and a portion thereof ejected, the plug means is drivento an activated position by the hydraulic forces acting thereon,whereupon the plug means will allow the liquid to be ejected from thesyringe but prevents any further refilling thereof.

In one embodiment of the invention, the fluid responsive plug meanscomprises a pair of axially aligned plug members disposed in the lowerchamber of the syringe. The upper plug number is axially moveable withinthe chamber by means of the hydraulic forces acting thereon duringliquid ejection from a raised inactive position to a lower activeposition. The upper plug member defines a fluid flow channel extendingaxially through the upper portion thereof, a conical recess extendingrearwardly from the underside thereof which communicates with the fluidflow channel at its apex, and an annular resilient flange which projectsoutwardly from the plug member and bears against the chamber wall. Theresilient flange cooperates with the surface of the chamber wall, firstto retain the plug member in the raised inactive position both prior touse and while air is aspirated into and ejected from the syringe throughthe flow channel in the plug member so that the syringe can be used asan air pump, and second, to secure permanently the plug member in thelower active position upon a volume of liquid having been aspirated intothe syringe and the plug member driven to said active position by theejection of said liquid.

The lower plug member is disposed within the chamber below the upperplug member and is axially moveable therein in an area below the upperplug member. The lower plug member defines a conical extension on theupper end thereof which extends into the conical recess in the upperplug member such that upon the upper plug member being driven to thelowered active position, the lower plug member allows air or liquid tobe ejected from the syringe but prevents return fluid channel fluid flowinto the syringe. The lower plug member also defines fluid flowpassageways in the side walls and the underside thereof to communicatethe hypodermic needle carried by the forward end of the syringe with thearea between the two plug members.

Prior to drawing fluid into the syringe through the hypodermic needlethereon, the upper plug member is held in the elevated position by theannular flange thereon bearing against the chamber wall. In thisposition, the piston disposed within the syringe above the upper plugmember can be moved upwardly and downwardly to draw and expel air intoand out of the syringe through the hypodermic needle. As the piston ismoved downwardly within the syringe, air is forced through the fluidflow channel in the upper plug member into the conical recess in thelower portion thereof, about the lower plug member and is expelledthrough the hypodermic needle. Upward movement of the piston effects anopposite passage of air into the syringe.

Upon inserting the hypodermic needle into a vial of liquid medicine andraising the piston within the syringe, the liquid is drawn upwardly intothe syringe about the lower plug member and through the upper plugmember into the area within the syringe between the piston and the upperplug member. As the piston is pushed downwardly to eject the medicine,the hydraulic force of the liquid acting against the upper surface ofthe upper plug member causes the upper plug member to move downwardly,whereupon the annular flange thereon passes over a radially extendingshoulder formed in the chamber wall into an enlarged diameter portion ofthe chamber and flexes outwardly. Concurrently, the liquid passesthrough the channel in the upper plug member into the recess in theunderside thereof, about the lower plug member and into and out of thehypodermic needle. The force of the liquid flow passing through theupper plug member and against the lower plug member, maintains the lowerplug member in lowered position within the chamber so that the conicalextension thereon will not block the downward flow of any of the liquidthrough the upper plug member to the needle.

Upon the upper plug member having been moved downwardly such that theannular flange thereon passes into the enlarged diameter portion of thechamber, any subsequent retraction of the piston will not effect furtheraspiration of fluid into the syringe regardless of whether or not all ofthe liquid has been injected through the hypodermic needle. Suchretraction of the piston will create a pressure drop in the syringebelow the piston, causing the lower plug member to move rearwardlyagainst the underside of the upper plug member which is prevented frommoving rearwardly by the abutment of the annular flange thereon with theaforesaid shoulder in the chamber wall. When the lower plug member abutsthe upper plug member the conical extension on the lower plug memberwill seal the lower end of the fluid flow channel extending through theupper plug member, rendering the syringe non-refillable and thusnon-reusable.

In a second embodiment of the invention, a single plug member isdisposed in a lower chamber of the syringe which is axially moveabletherein by the hydraulic forces acting thereon during liquid ejectionfrom a raised inactive position to a lower active position. The plugmember defines a resilient flange which projects outwardly from theundersides thereof and cooperates with the interior wall surface of thechamber, first to retain the plug member in the raised inactive positionboth prior to use and while air is aspirated into and ejected from thesyringe so that the syringe can be used as an air pump, and second, toretain the plug member in the lower active position upon a volume ofliquid having been aspirated into the syringe and the plug member drivento said active position by the ejection of said liquid. In the raisedinactive position air is free to pass through the lower syringe chamberand about the plug members therein by means of fluid flow passagewaysformed in the upper perimeter portion of the plug member and in thelower surface of the resilient flange. In the lower active position, anywithdrawal of the piston in an effort to draw additional liquid into thesyringe will draw the plug member rearwardly within the chamber causinga sealing engagement of the resilient flange with a shoulder formed inthe chamber wall, preventing further aspiration of fluid into thesyringe and thereby rendering the device non-reusable.

IN THE DRAWINGS

FIG. 1 is a sectional view of the hypodermic syringe of the presentinvention as fluid is being drawn therein.

FIG. 2 is a sectional view of the lower portion of the hypodermicsyringe of the present invention showing the relative positioning of theplug members as liquid is being ejected from the syringe.

FIG. 3 is a partial sectional view of the lower portion of thehypodermic syringe of the present invention during an attemptedrefilling of the syringe.

FIG. 4 is a partial sectional view of lower portion of a modified formof the present invention.

FIG. 5 is prospective view of the lower plug members of the presentinvention.

FIG. 6 is a view taken along line 6--6 in FIG. 5.

FIG. 7 is a sectional view of a second embodiment of the presentinvention showing the position of the plug member prior to liquid beendrawn therein.

FIG. 8 is a partial sectional view of the lower portion of a secondembodiment of the present invention showing the position of the plugmember as liquid is being ejected from the syringe.

FIG. 9 is a partial sectional view of the lower portion of a secondembodiment of the present invention showing the position of the plugmember during an attempted refilling of the syringe.

FIG. 10 is a perspective view of the plug member of the secondembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The non-reusable hypodermic syringe 10 of the present invention includesa cylindrical barrel 12, a double-landed piston 14 and a plunger rod 16for use in reciprocating the piston 14 within barrel 12. Rod 16 definesa gripping flange 17 on the rearward end thereof and is preferablysecured to the piston 14 at its forward end by means of an enlargedtapered portion 18 held within a correspondingly configured recess 20 inthe rearward portion of piston 14. An upper plug member 22 is disposedbelow piston 14 within a chamber 23 defined by the lower portion ofbarrel 12 and a barrel cap 26. A lower plug member 24 is disposed inchamber 23 axially below plug member 22. Barrel cap 26 is permanentlyaffixed to the lower end of barrel 12 by heat bonding of a suitableadhesive, and a hypodermic needle 28 is permanently secured within ahollow extension 30 extending from barrel cap 26.

The syringe barrel 12, which is preferably constructed of a transparentdurable plastic material, defines an outwardly extending radial flange32 at the upper end thereof, a constant diameter upper portion 34 withinwhich piston 14 is axially translatable, a lower portion 36 whichdefines the interior wall of chamber 23 and an intermediary annularhorizontal end wall 38. A stop ring 39 is provided at the rearwardmostend of barrel 12 to prevent withdrawal of the piston from the barrel soas to preclude access to the interior of the barrel.

The upper plug member 22, preferably constructed of a rubber materialincludes an enlarged diameter upper portion 40, a body portion 42, ashoulder 44 disposed therebetween, a resilient annular flange 46extending outwardly from the lower end thereof, and a fluid channel 48extending axially therethrough and communicating with a conical recess50.

Shoulder 44 is adapted to abut a first shoulder 52 defined by the lowerportion 36 of barrel 12 in chamber 23 to limit the downward movement ofthe upper plug member 22 within chamber 23. As seen in FIG. 1, theresilient annular flange 46 extends outwardly and upwardly from thelower end of plug member 22 such that when the upper plug member 22 isin the raised inactive position, flange 46 flexes outwardly and bearsagainst side wall 54 in chamber 23 and abuts a second shoulder 56defined by the lower portion 36 of barrel 12 in chamber 23 to limit theupward travel of plug member 22 within chamber 23. The lower portion 36of syringe barrel 12 also defines a third interior shoulder 58 inchamber 23 located between side wall 54 and a lower side wall 59disposed about an enlarged diameter portion 60 of chamber 23 adjacentthe lower end 62 of barrel 12.

The lower plug member 24, also preferably constructed of a rubbermaterial, is disposed axially below upper plug member 22 in chamber 23.Lower plug member 24 defines a lower base portion 64, and a conicalextension 66. Lower base portion 64 defines a diameter less than theinner diameter of end cap 26 to allow fluid flow therebetween butgreater than the inner diameter of barrel 12 at the end 62 thereof so asto be axially translatable within chamber 23 between the lower end wall63 of barrel cup 26 and the lower end 62 of barrel 12. Extension 66projects upwardly from base portion 64 into the recess 50 in the upperplug member 22, and is configured so as to abut and mate with theconical side wall of recess 50 upon the plug members being forcedtogether so as to seal the lower end of fluid channel 48 in the upperplug member 22 to prevent fluid flow therethrough. As seen in FIG. 5 and6, fluid passages 68 are formed in the side and bottom walls of thelower plug member 24 so as to communicate the hypodermic needle 28 withthe portion of chamber 23 above the lower plug member 24 so as to allowfluid flow therebetween about lower plug member 24 when the lower plugmember is in its lowermost position against end wall 63 of barrel cap26.

Prior to any fluid being drawn into the syringe 10 through needle 28,the upper plug member is in the raised position in chamber 23 asillustrated in FIG. 1. In this position, the syringe 10 can be used asan air pump by means of piston 14, as the downward movement of piston 14will force air in barrel 12 through chamber 23 via the channel 48 in theupper plug member 22, about the lower plug member 24 and out needle 28,but will not create sufficient force against the upper end of plugmember 22 to move the plug member downwardly within chamber 23.Accordingly, if one needed an injection of, for example, 20 units ofmedicine, the user would retract the piston to a point slightly abovethe 20 unit mark on the barrel (not shown), insert the needle into theliquid medicine in the vial through the standard seal thereon, push thepiston back down to wall 38 to drive a volume of air slightly in excessof 20 units into the vial and retract the piston to the point slightlyabove the 20 unit mark to draw slightly over 20 units of medicine intothe barrel of the syringe. The syringe is then held in an invertedposition with the needle pointing upwardly, and the piston is againpushed back to the 20 unit mark, ejecting the excess medicine from theneedle as well as any air remaining in the syringe. The syringe is thenready to inject the measured 20 units of medicine into the user and, dueto the air pumping capability of the syringe, the interior of the vialof medicine is left at atmospheric pressure.

As the piston 14 is then driven downwardly during the injection, theliquid medicine is forced through the channel 48 in the upper plugmember 22, about the lower plug member 24 through the fluid passages 68therein and into and out of the hypodermic needle 28. Concurrently, thehydraulic forces created during the injection by the liquid being drivenby the piston act on the effective upper surface areas of the upper andlower plug members 22 and 24 to drive the upper plug member 22downwardly within chamber 23 to the position illustrated in FIG. 2 andthe lower plug member 24 downwardly against the lower end wall 63 of capmember 26. As the resilient flange 46 on the upper plug member 22 passesthe shoulder 58 in chamber 23, the flange flexes outwardly within theenlarged diameter portion 60 of chamber 23 and bears against the sidewall 59 of the enlarged diameter portion 60 of chamber 23. The upper andlower plug members will then remain in this position during theinjection. If, either after the completion of the injection or duringthe injection, one were to attempt to retract the piston 14 within thebarrel to aspirate more liquid into the syringe through needle 28, theresultant vacuum created within the barrel below the piston 14 woulddraw the upper and lower plug members upwardly to the positionsillustrated in FIG. 3. The upward travel of upper plug member 22 duringsuch an attempted refill would be limited by the abutment of the flange46 thereon with shoulder 58 in chamber 23, while the conical extension66 on the lower plug member 24 would be drawn into recess 50 and againstthe upper plug member, sealing the lower end of channel 48, and therebypreventing any fluid flow upwardly through the upper plug member 22. Thesealing engagement of flange 46 with the side wall 59 and shoulder 58 inchamber 23 would prevent any upward fluid flow about the upper plugmember 22. Accordingly, prior to drawing any liquid into the syringe 10,the piston can be moved upwardly and downwardly within the barrelwithout effecting deactivation of the syringe. However, once a liquidhas been drawn into the syringe and the piston moved downwardly a slightdistance sufficient to cause a translation of the lower plug member 22 adistance equal to axial length of side wall 54, flange 46 clearsshoulder 58, flexes outwardly and the Syringe is rendered inoperative,other than to eject the fluid contained therein without any affirmativeaction or other cooperation of the user.

While the above mechanism provides a non-reusable syringe which cannotbe refilled by the user, to dissuade one from continuing to try to makethe device function properly or from even attempting to disassemble thesyringe to remove the disabling plug members, the enlarged tapered end18 of the plunger rod 16 by which the rod is secured to piston 14 isconfigured to separate from the piston in the event one were to attemptto draw additional fluid into the syringe. Once the upper plug memberhas been driven forwardly to the lower position by injection of liquidfrom the syringe, any subsequent retraction of the piston within thebarrel will create a vacuum within the barrel between the piston andupper plug member as a result of the sealing engagements of the upperand lower plug members 22 and 24. As this vacuum increases with theretraction of the piston, the opposed forces acting on the upper andlower ends of the piston will cause the enlarged tapered end 18 of theplunger rod 16 to be pulled out of the tapered recessed 20 in thepiston, separating the rod 18 from piston 14 and making the destructionof the syringe readily apparent to the user so as to dissuade the userfrom any further attempts to use the device or to try to disassemble andreconstruct the syringe. In addition, a washer 80 could be provided onbarrel wall 38 which extends slightly over the upper perimeter surfaceof upper plug member 22 to prevent one from being able to somehow pullthe plug members from the barrel. Alternatively, end wall 38 could beextended radially inwardly to provide the same function.

FIG. 4 illustrates a modified form of the above described embodiment ofthe invention which, while operating in the same manner as the priorembodiment, differs slightly from the prior embodiment in theconfiguration of the upper plug member 122 and the lower portion 136 ofthe syringe barrel 112. In the second embodiment, the upper plug member122 which, like the upper plug member 22 of the prior embodiment, ispreferably constructed of a resilient rubber material, comprises anenlarged diameter upper radial flange portion 140, a central bodyportion 142 of reduced diameter, an enlarged lower radial flange portion146, a central axial fluid channel 148, and conical recess 150. Theenlarged lower flange portion 146 is sized so as to be in slightcompression against a side wall 154 defined by barrel 112 to retain plugmember 122 in place prior to ejection of liquid therethrough and suchthat upon plug member 122 being forced downwardly by the hydraulicforces acting thereon during liquid ejection, the enlarged lower flangeportion 146 will flex outwardly upon clearing a shoulder 158 defined inthe lower portion of the barrel to maintain the plug member 122 in alowered position and, in combination with a lower plug number 24,disable the syringe from being able to draw additional liquid therein inthe same manner as described above with respect to the prior embodiment.

In addition, the syringe barrel can be modified-as seen in FIG. 4 toprovide an inwardly extending radial flange 138 having an uppercurvalinear surface 139. Flange 138 prevents withdrawal of the plugmembers from the barrel, while the upper curvalinear surface 139 thereonwill assist the user in aligning any air bubbles in the medicine withthe fluid flow channel 148 in the upper plug member 122 for removalthrough needle 28. The configuration of flange 138 can be used witheither embodiment of the invention. However, when using flange 138, thelower surface of the piston should be corresponding configured as seenat 114 in FIG. 4.

FIGS. 7-10 illustrate a second embodiment of the invention which employsonly a single plug member to render the syringe non-reusable. Thesyringe 200 of the second embodiment differs from syringe 10 soley inthe lower portion thereof and accordingly only the lower portion ofsyringe 200 will be described. Syringe 200 includes a barrel 202defining a constant diameter portion 204 within which a piston 206 isdisposed. Piston 206 is preferably provided with a curvalinear lowersurface 208 and is carried by plunger rod 210. Barrel 202 defines alower syringe chamber 211, an annular shoulder 212 disposed in chamber211 and a curvalinear lower surface 214 having a concave position 216terminating a flat horizontal surface 218 having a central opening 220therein. A barrel cap 222 is carried by the lower end of barrel 202 anda hypodermic needle 223 is carried by barrel cap 222 such that itcommunicates with chamber 211 through the opening 220 in barrel cap 222.

An annular stop member 224, preferably constructed of the same durableplastic material as barrel 202, is affixed to the interior surface ofbarrel 202 so as to define the upper end of chamber 211. Stop member 224defines a curvalinear upper surface 226 adapted to mate with thecorrespondingly configured lower surface 208 of piston 206, a centralopening 228 therein, a flat horizontal lower surface 230 and an annulardepending portion 232 which abuts shoulder 212 in chamber 211.

A plug member, preferably 234 constructed of a rubber material, isdisposed within chamber 211, and defines: an upper portion 236, a curveddownwardly tapered portion 238 which is disposed below portion 236 andinwardly spaced from the perimeter thereof, and a resilient lowerannular flange 240 disposed below tapered portion 238. The outerdiameter of the upper portion 236 of plug member 234 is less than theinner diameter defined by the interior side wall 242 of stop member 224so as to allow fluid flow therebetween. For reasons which will becomereadily apparent, the diameter of lower flange 240 in the unflexedstate, as seen in FIG. 9, is greater than the inner diameter defined byinterior side wall 242. As seen in FIG. 9, fluid passageways 244 and 246are provided in the upper surface of plug member 234 and in theunderside of resilient flange 240 respectively to allow for fluid flowabout the plug member. Alternatively, the fluid passageways could beformed in the lower surface 230 and depending portion 232 of stop member224 and in the lower surface 214 of barrel 202 to allow for such flow.

In the raised inactive position illustrated in FIG. 7, flange 240 isdisposed substantially adjacent the tapered portion 238 of the plugmember 234, flexes outwardly and bears against the side wall 242 so asto retain the plug member 234 in the raised inactive position. Prior toany liquid being drawn into the syringe 200, the syringe can be used asan air pump, as with the prior embodiment, as air is free to passthrough chamber 211 about plug member 234 via fluid passageways 244 and246 without moving the plug member. Upon a liquid being drawn into thesyringe and the piston 206 being moved downwardly, the liquid is forcedthrough chamber 211, about plug member 234 through the fluid passageways244 and 246 therein and out needle 224. Concurrently, the hydraulicforces acting on plug member 234, force the plug member downwardlywithin chamber 211, causing flange 240 to move past the lower end 248 ofstop member 224 and against the lower surface 214 of the barrel 202 tothe position illustrated in FIG. 8. As soon as flange 240 clears end248, it flexes outwardly in the enlarged diameter portion 241 of chamber211 and the plug member 234 is in the active position. Thereafter,withdrawal of the piston 206 in an effort to draw more liquid into-thesyringe will cause the plug member to be drawn rearwardly within chamber211 such that the upper surface 249 of flange 240 will abut the lowerend 248 of stop member 224 and prevent fluid passage therebetween asseen in FIG. 9, thereby rendering the syringe non-reusable.

The sensitivity of the embodiments of the non-reusable syringe of thepresent invention is determined by the axial distance the flange 46 onthe first embodiment, the enlarged flange portion 146 on the modifiedform thereof, or the flange 240 on the second embodiment, travels beforeflexing outwardly to secure the plug member in its lower activeposition. For example, in the second embodiment this distance depends onthe axial length of the depending portion 232 of stop member 224. Byincreasing this axial length, or the length of side wall 54 in the firstembodiment, the syringe could be designed to allow for ejection of asmall amount of liquid prior to the plug member being locked in itslower active position. This would provide the user with some flexibilityin the event of inadvertly ejecting slightly too much liquid during airexpulsion prior to an injection as the user could then draw a portion ofan additional unit of medicine into the syringe to correct the error.

Various other changes and modifications may be made in carrying out thepresent invention without departing from the spirit and scope thereof.Insofar as these changes and modifications are within the purview of theappended claims, they are to be considered as part of the presentinvention.

We claim:
 1. A single use disposable hypodermic syringe comprising:ahollow elongated barrel portion; a piston in sealing engagement with andaxially translatable along said barrel portion; a piston rod secured tosaid piston and projecting from one end of said barrel portion for usein reciprocating said piston; a walled chamber disposed proximate theother end of said barrel portion and defining an annular shouldertherein; and a plug member disposed in said chamber and movable thereinfrom a rearward position to a forward position in response to liquidbeing forced against said plug member by said piston, said plug memberdefining a resilient retaining flange having a forward plug surface atthe forward end thereof, a body portion disposed rearwardly of saidretaining flange and a plurality of fluid flow passages for the passageof fluid through said chamber to and from said barrel portion when saidplug member is in said rearward position, said fluid flow passages beingdefined by elevational discontinuities in said forward surface of saidflange, said flange retaining said plug member in said rearward positionprior to liquid being drawn into said barrel portion and allowing avariable volume of air to be drawn into and ejected from said barrelportion through said passageways when said piston is reciprocated withinsaid barrel portion during the use of said syringe as an air pump and,upon liquid being drawn into said barrel portion through said fluid flowpassageways and at least a portion thereof ejected from said barrelportion, said plug member cooperates with said shoulder to seal saidpassageways in response to said piston being retracted within saidbarrel portion toward said one end thereof, thereby preventing anyadditional liquid from being drawn into said barrel portion through saidpassageways and rendering said syringe non-reusable.
 2. The syringe ofclaim 1 including a plurality of fluid flow passageways defined by saidbody portion of said plug member and communicating with said fluid flowpassageways defined by said flange when said plug member is disposed insaid rearward position.